differential vent

P

Preacher

Senior Member
one or two of the local club members have vented their differential and claim it leaks less or not at all after tours. here is how did mine. just did it, so will need to make a determination later.
 

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BRENT in 10-uh-C said:
Why do you feel it is needed?
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assuming there is heat generated, there is a pressure build (however small) that will cause oil to be pushed out of places it may otherwise not seep. the vent has a tube going up & forming a loop. just like on a modern vehicle. again, I will be installing soon as mine does leave it's mark while parked in my garage. Then I will be able to report back if I benefited from the change
 
OK, for the sake of a good discussion, let's just say the housing reached a temperature of 150° during operation. It may not even be that hot because I know that I can touch the housing without causing a 1st degree burn even after touring quite a few miles. Now if you consider the amount of air that is in both axle housings and torque tube that does not have grease in it, ...plus the manner in which the housing seals work, I cannot believe there is any chance of pressure building up inside from the limited amount of grease that is inside.

To prove me wrong, you could take a vacuum gauge that also measures minimal amount fo pressure and install it in the vent hole to see if any pressure builds, but again, I'm struggling to believe there is enough pressure inside to push grease out of a gasketed area.
 
I agree with Brent on this one. Excess leakage is most commonly caused by a bad seal in the torque tube overfilling the rear axle hsg, not bad venting capabilities. Just an observation.
 
How about installing the special vent bolt, connect a small hose to it and blow into the hose and see if you can build pressure. If you can't build pressure that will hold for a metter of seconds, then no added vent should be needed.
 
As a point of interest, some pre 49 V8 rears had factory vents and some did not. Why this is I have no idea. I have a 47 car rear without, and a 47 open drive pickup rear with.
 
these are ALL very good observations and opens up quite a few 'projects' for me to pursue! == just obtained an infrared thermometer to check the exterior temps generated on my drums and differential.
 
A stock Model A will vent the differential up the driveshaft and into the trans where it is vented in the shift tower.

Adding an overdrive could eliminate that path and require a vent.

Open drive line cars do not have that path so come from the factory with a vent.
 
If you are using original type leather seals there is no problem with venting, the leather type seals could allow some air movement, it is not the expansion of the oil that creates the pressure, is is the change in air pressure from the change of air temperature.

When you use "modern" seals --- for the driveshaft,and axle seals, then add a sealed bearing tor the trans the venting path becomes blocked, or at least much more restricted.

A modern rubber lip seal with a spring to hold the lip to the shaft is designed to not allow any passage of liquid ---or air.
 
Kurt in NJ said:
If you are using original type leather seals there is no problem with venting, the leather type seals could allow some air movement, it is not the expansion of the oil that creates the pressure, is is the change in air pressure from the change of air temperature.

When you use "modern" seals --- for the driveshaft,and axle seals, then add a sealed bearing tor the trans the venting path becomes blocked, or at least much more restricted.

A modern rubber lip seal with a spring to hold the lip to the shaft is designed to not allow any passage of liquid ---or air.
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True, ..........in the perfect world, --which IMO, the Model A rear end is not.

To me, the expansion of the heated air would likely cause the pressure to rise to less than 3 or 4 lbs.(??) Due to the axles being loose within the housing, there is not a 100% way to make that seal just because the hub bearing allows axle movement as the vehicle bounces which would likely "burp" any excess pressure. From that moment on, the warm air has been "normalized" to atmospheric pressure and there should be no reason for leaks. I would also think that if the mating surfaces were restored by using a flat file where there is a good seal between the gasket shims, those should be able to withstand the minimal amount of pressure created by the heated air.

One other point I would make is this, if I remembering correctly, where the differential housing mates to the torque tube does not use a gasket there. Granted most folks install a gasket, --or do as we do and put a thin bead of Permatex there to seal it, but if someone was genuinely concerned, a thin file mark between the mating surfaces would allow the air to escape. Just my thoughts............
 
"As a point of interest, some pre 49 V8 rears had factory vents and some did not. Why this is I have no idea. I have a 47 car rear without, and a 47 open drive pickup rear with. " Reason...the '42-48 open drive banjo and the '49 up rears were sealed, with a seal where pinion reached the world. Model A and up to '48 passenger lacked a seal there and the A even lacked a gasket. Everything eventually reaches a seal, at axle ends and behind the driveshaft U-joint area, but even if you assume air molecules too dumb to find their way out of the primitive maze there is scads of expansion room into areas that are without the small heating effects of the gears.
 
I like the idea of replacing one of the top bolts on the differential housing with a bolt that is drilled. Add a small 90 degree fitting and a piece of tubing held up high. That should be a simple way to see if a vent is truly needed and gets you there without the possibility of ending up with metal shaving inside the housing unless you are making the hole with the assembly all apart. Sure would like to hear updates later on the findings.
 
I could see a car wanting to vent while driven due to axle movement.

The problem is the cars sit for extended periods of time. In some areas it is not uncommon to see 60* change in temps from day to night. So they would not get the chance to burp.

Keep in mind these same temp variations are responsible for the flavor in your favorite Whiskey. They push the liquid in and out of the wood on the sides of the barrel.

The temp changes are also part of why the hydraulic brakes fail. As the fluid is pushed and pulled past the seals.

A few notes about the drive shaft housing gaskets.

From what I have read in the Ford manuals, the gasket for the V8 housings was only to be used when you put a shim on the bearing to set the pinion. You are to put a matching gasket shim on the housing to banjo. When anything is used on the A's you are allowing slight pinion movement. The housing locks the pinion double race in place.
 
I'll bet if you stuck a sealed hose in the filler plug area and blew into to you would not be able to make pressure. Like Brent said it does not get hot. Maybe warm. But not hot enough to make pressure. Any 'pressure leaks past the roller bearings and seals and up the sealed driveshaft toward the U-joint.

skip.
 
claim it leaks less or not at all after tours. QUOTE said:
The entire premise is based on the belief that built up pressure from heat expanded air is pushing lube out AFTER tours. My thought is that, by the end of the tour the pressure has equalized, and as the rear end cools....it would be creating negative pressure and would not leak (from pressure). Its leaking cause the banjo seals are not good, not from expanded air pressure. Just my opinion.
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All of us that know Preacher love and respect him. He is about 6'21" and when he is at a Model A workshop tinkerin' with his distributer machine and all of us "normal sized folks" around him he is in dog heaven. He drives that Model A of his everywhere If you see a big guy with "Clergy" on his plates driving where he shouldn't it's got to be Preacher. His saying suits him to a "T" "if it ain't broke fix it til it is". Hey Preacher, stop foolin' around with the @#$%^& rear end and make sure the valves don't fall out in Whitewater, again.
 

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